Frequency translating circuits



Dec. 27, 1927.

H. S. BLACK FREQUENCY TRANSLATING CIRCUITS Filed Nov. 30. 1926 3 Sheets-Sheet l Dec} 27; 1927.

. 1,653,887 H. S. BLACK- FREQUENCY TRANSJLATING CIRCUITS Filed Nov, 50. 1926 5 Sheets-$heet 3 MODULA ran GAIN-MILES O\ Mam/mum GAIN-MLES 0\ & Warm/a 5. fi/krc/r A/7amey Patented Dec. 27", 1927.

UNITED STATES; PATENT OFFICE.

HAROLD 8. BLACK, 01'! NEWARK, NEW JERSEY, ASEIGNOR TO BELL-TELEPHONELABO- RATORIES, INCORPORATED, 031 NEW YORK, N. Y., 'A CORPORATION OF NEW YORK.

FREQUENCY TRANSLATING CIRCUITS.

Application filed November 30, 1926. Serial No. 151,678.

This invention relates to frequency trans lating circuits, and particularly to modulators and demodulators for carrier current telephone systems.

An object oi? the invention is toincrease the efficiency and stability of operation of such circuits.

A further object of: the'invention is to provide a frequency translating circuit which will operate with high ellicicn'cy and stability at high power levels.

In. the copending application of H. S.

Black Serial No. 58,744, filed September 26,

1925, there is described and claimed a trans lating circuit which employs an impedance element in the input circuit for establishing a grid biasing potential proportional to the potential of-the carrier ave. Such an arrangement makes the operation of the circuit very stable. However, the use of the impedance element causes the grid biasing potential to be, to some extent, also dependent upon the potential oi the signaling wave. As a result, when this type of circuit is operated as a self-oscillator as in the type described and specifically claimed in the above application, high levels of signaling energy will increase the bias on the grid and consequently lower the voltage oi the generated carrier wave, so that with a given type of discharge device there is a limit to the energy level at which the circuit can be satisfactorily operated.

In accordance with a feature of this invention, a separate source of carrier is used in a translating circuit in which the grid biasing potential is obtained by means of an impedance element in the input circuit, so that the voltage of the carrier wave is independent of the voltage of the signaling wave, thus permittingthe use o1 signaling waves of higher energy levels and allowing much greater. power output.

In one embodiment, this invention comprises a modulator or demodulator circuit, particularly suited for use in a carrier signaling system in which the output modulated side band is very close in the frequency scale to the input signaling waves. The translating circuit is of the balanced type employing two electric discharge devices having their plates directly connected together. A. resistance shunted by a condenser is employed in the input circuit for giving a grid biasing potential. The condenser provides a path of low impedance for the alternating current components, and tends to maintain the biasing potential during the portions of the cycle when no current is flowing in the grid circuit. The ofit'ect of this condenser and the secondary windings of the input transformer which are'in parallel opposing relationtor grid current modulation products prevents those products from being transmitted to'the output circuit. The t animals of the source of carrier current and of the incoming speech or side band are connected to the input of the translating circuit in conjugate relation, so that the carrier wave, the frequency of which is of the same order of magnitude as that of the signaling waves, will not be impressed upon the signaling circuit, thus greatly reducing the requirements of the input lilter. The effect 01 the manner of connection of the output circuit to the discharge devices is to suppress both the signaling and carrier waves, thus greatly reducing the requirements of the output fil tors. The output circuit is preferably given an impedance which is equal to the output impedance of the tubes for the side band to be transmitted andwhich is zero for all other frequencies, particularly those of the signaling and carrier waves and the side band to be suppressed, including the various harmonics and higher order modulation products.

These and other objects and features of the invention will be more readily understood by reference to the following detailed description in connection with the drawing, in which: Fig. 1 shows diagrammatically one embodiment of the invention;

Fig. 2 shows graphically the characteristic curve of a typical discharge device illustrating the method of operation of the circuit; i

Fig. 3 shows graphically the ratio of carrier peak voltage to the component of carrier effective for modulation;

Fig. l shows diagrammaticallya second embodiment of the invention; and V I Figs. 5 and 6 show graphically the ratio of input to gain, illustrating the stability of the system of this invention, as compared to that of a circuit employing a gridbiasing battery. i I

Fig. 1 shows a translating circuit embodying this invention. This circuit may be used as either a modulator or a demodulator, depending upon the character of the input current. Two discharge devices 11 and 12 areponnected .in push-pull relation in the usual manner to an input transformer 13 and an output transformer 1a. Carrier current is supplied from an oscillator 15 to the input circuit through a transformer- 16 connected in the common cathode lead. Connected in this lead, in series with the secondary of the transformer 16 is a resistance 17,, shunted by a condenser 18. Plate current is supplied from av battery 19 connected in the plate circuit in series with a retard coil 20 and shunted by condenser 21. A resistance'23 and condenser 2a in parallel are connected in shunt to the secondary'of the transformer 13.

A filter 22 is connected to the terminals of the secondaryof the output transformer it for suppressing all but the side band to be transmitted in the case of a modulator circuit or the signaling wave in the case of a demodulator. F or the best operation, the input impedance of the filter 22 is matched through the transformer to the impedance of the discharge devices for the frequency to be transmitted (that is, the side band or signaling wave) and-made substantially zero for the frequency of all other waves, including the harmonic frequencies and higher order modulation products.

The operation of the circuit as a inodulator will be described, the operation as a demodulator being essentially the same, ex cept for the character of the input current and the useful output product. The operation is similar in certain respects to that disclosed in U. S. Patent to Carson 1,343,306 of June 15, 1920. As described in the application No. 58,744, referred to above, the es sential difierence between the operation of this general type of circuit. and that described in the Carson patent is the use of an impedance element for obtaining agrid biasing potential proportional and practically equalto the peak voltage of the carrier wave. The operation of this circuit is essentially similar to that of the. circuit described in that application, except that the use of a separate source of carrier permits the circuit to be operated at higher energy levels.

The use of resistance 17 for obtaining the bias is best explained by reference to Fig. 2. In this figure, A is the grid potentialspace current characteristic of a typical electric discharge device adapted to be used in the circuit of Fig. 1. In the type of circuit described in the Carson patent, the tubes are operated about some point such as Q, giving a normal space current which is varied between some minimum and maximum values by the potentials impressed upon the grid, and any change in the operating point up sets the adjustment of the circuit, to some extent. In this method of operation, the modulated output is proportional to a first ap proximation to the product CV (carrier times voice) and hence for best operation (1 plus V should be constant with C=V for maximum eiiiciency, as regards output, so that the carrier output is varied not only by the lined potentials of the tube but by variations in the peak voltage of the carrier.

According to the present invention, the circuit is operated at point P well olf the characteris ic curve and of such value that the carrier wave 1 drives the grid slightly positive once during each positive half-cycle, as indicated by the shaded portions of the carrier wave. lhis slight positive grid potential causes rectification to take place in the grid circuit, sending a current through the resistance 17 and setting up an effective negative polarizing potential, due to the potential drop across this resistance by which the operating point is maintained. The condenser 18 serves to maintain this polarizing potential during the intervals in which the grid is not driven positive. The resistance 17 and the condenser 18 are so proportioned that only a slight positive grid potential is required to give this eiiective bias.

If the voltage of the carrier wave increases, for example, to a value such as indicated by wave l, the grid is driven more positive, sending more current through the resistance 17, thus building up a negative bias the value of which increases rapidily with increases in carrier voltage, with the result that the effective grid bias becomes sutiiciently large to prevent the peak voltage of the wave from making the instantaneous grid potential substantially any more positive than'l'or the smaller amplitude wave. 1. In other words, the operating point is changed in accordance with the voltage of the carrier wave so that with a wave of different voltage, such as is shown at 4-, the ciruit is effectively operated about another point, such as R, giving positive peaks of substantially the same value as with the nor-- mal carrier voltage indicated by curve '1.

lhe total instantaneous space currents corresponding to the two impressed waves 1 and d are shown at 1' and i, respectively. The amplitude of these waves is the same showing that with the circuit of this invention the carrier output is substantially independent of the carrier peak voltage. The only essential difference in these two wave forms is that the waves'a corresponding to the higher carrier voltage, are somewhat sharper than the waves 1, the harmonics being slightly more pronounced with the type l than with 1. As regards the sum total of the useful side band power outputs, these harmonics are important in that, first, they contribute in the form of higher order products to the (l-tVor C- V, as the case may circuits remaining unchanged, then the tube resistance is increased and the side band power is reduced. However, the eificiency of the.circuitwith properly designed impedance relations is largely dependent upon the amount of fundamental present.

Fig. 3 shows the relation between the carrier peak voltage and the component which is effective for modulation purposes, that is, the fundamental for various values of carrier peak voltage.

The condenser 1.8 tends to maintain the biasing potential during the intervals when no current is flowing in the grid circuit and in addition provides a path of low impedance for the carriercurrent. x

As in the Carson type circuit the useful modulation takes place in the plate circuit. However, the effect of the resistance 17 will be to tend to produce grid current modula tion products, and since these are of opposite phase to the useful plate current modulation products it is desirable that they be suppressed. The effect of condenser 18 is to reduce the value of these grid current modulationproducts. Their effect is still further reduced by the condenser 2a which is ofsuch a value as to have a high impedance to signaling currents and low i1npedancevto these modulation products. This condenser therefore, provides a low impedance path between the points at which the side bands resulting from grid current modulation tend to occur and thereby prevents the amplitude of these products from reaching a value which would materially reduce the amplitude of the useful plate current products.

The effect of the signal currents'on the grid circuit is to alter the biasing potential only slightly. However, the effect of these currents is in the direction of cutting down the gain, since their effect in the plate circuit is substantially independent of their voltage, as is evidenced by the fact that the curves 1 and 4- have about the same amplitude, notwithstanding the difference in the impressed potential. Whatever grid biasing effect the speech waves have, therefore,

will be in the direction tending to maintain the space current constant. rather than changing it, while modulation, of course, re-

quires that it shall be changed in accordance with the impressed speech. This slight sacrifice in gain, however, enables the circuit to accommodate greater load variations, since when the speech and carrier waves are both applied the grid bias is slightly greater than when the carrier alone is applied.

Figs. 5 and 6 show very clearly the increase instability of operation which is obtained with the circuit of this invention. In both figures, input current at 1000 P. S. is plotted against modulator gain, and'the various curves are for different values of carrier voltage as indicated. Fig. 5 shows the characteristics of the Carson type circuit employing a grid biasing battery and Fig. 6 shows the characteristics of the circuit of this invention. In Fig. 5 it will be noted that as the voltage of the carrier Wave is varied by only a few volts from'the optimum value of 30st volts the operation becomes very erratic, while in Fig. 6 there is practieably no change in the operation When the oltage is varied from 44 to 75 volts.

Fig. l shows a modification of the translating circuit of this invention which is well suited for use as a modulator or demodulator in a carrier signaling'system in which the useful side band is very close in "the frequency scale to the signaling wave,

such as might be the case in a cable circuit on which a carrier telephone system is to be superfn'iposed. In this circuit two electric discharge devices 31 and 32 are connected together in the manner describedin Hartley Patent No. 1,419,562, of June 13, 1922. In this circuit a signaling wave to be modulated, or a side band to be demodulated, and

a carrier wave are impressed upon the grids of the devices 31 and 32 through an input transformer 33. The grids are connected respectively, to the-two terminals of the sec- 'ondary winding and the cathodes are con nected together and to the mid-point of this winding. A resistance 37 shunted by a con-- denser 38is connected in the common cathode lead.

The anodes or plates are connected together and through the primary of an out- )ut transformer 34; and a. plate battery 39 to the cathodes. A filter is connected to the secondary winding of the transformer 34: to transmit the desired side band and suppress the other waves, As described'in The carrier wave is supplied from an oscillator 40 through a transformer 41, the

from" being impressed on the circuit d2.

In other respects the operation of this circuit 1S analogous to that of Fig. 1. The rcsistance 37 and condenser 38 act in the same wayito produce an effective biasing poten- .tial.

. Thejgrid current modulation products are prevented from being effective in the output circuit by the secondary winding of transformer 33 offering a practically zero impedance to them. This is due to the fact that, since the two devices are in effect operating in parallel for the transmission of these products, the two halves of the secondary winding of transformer 33 are in parallel opposing relation. This together with the fact that the impedance of con denser 38 is'very low for such products will make the input impedance of the two devices substantially zero to these products so that they will not be transmitted to the output circuit. tion is in effect suppressed.

If the characteristics of the input transformer or other factors, make it desirable a condenser may be connected in shunt to the secondary winding, as in the circuit of Fig. l, to further aid in suppressing the harmful effects of grid current modulation. Other modifications may be made without departing from the spirit of the invention as defined in the appended claims.

lVhat is claimed is:

1. A frequency translating circuit com prising a discharge device, having an anode, a cathode and 'a control electrode, an input circuit therefor, a separate source of carrier waves and asource of signaling waves connected to said input circuit, and means in said inputcircuit for applying to the control electrode an effective negative biasing potential dependent upon the value of the voltage of the carrier Wave.

2. A frequency translating circuit according to claim 1 in which said means comprises a resistance and a condenser in parallel.

Afrequency translating circuit according to claim 1 having means in the input circuit for effectively preventing grid current modulation.

4. A frequency translating circuit comprising an electric discharge device having Thus, grid current modulaaccess? an anode, a cathode and a control electrode, an input circuit therefor, a separate source of carrier waves and a source of signaling waves connected to said input circuit, an impedance element in said input circuit for applying to the control electrode an effective negative biasing potential proportional to the drop across said element produced by current from said source of carrier Waves, and means in said input circuit for effectively preventing grid current modulation.

5. A frequency translating circuit comprising a pair of electric discharge devices each having an anode, a cathode and a control electrode, an input circuit therefor having a common portion connected to the cathodes of said devices and a portion individual to the control electrode of each of said devices, a separate source of carrier waves and a source of signaling Waves connected to said input circuit, and means in said input circuit for applying to the control electrode an eifective negative biasing p0- cntial proportional to the peak voltage of the carrier Wave.

6. A frequency translating circuit comprising a pair of electric discharge devices each having an anode, a cathode and a control electrode, an input circuit therefor having a common portion connected to the cathodes of said devices and a portion individual to the control electrode of each of said devices, a separate source of carrier Waves and a source of signaling waves connected to said input circuit, an impedance element in said input circuit for applying to the control. electrode an efiective negative biasing potential proportional to the potentialdrop across said element produced by current from said source of carrier waves, and means n said input circuit for effectively preventing grid current modulation. 7. A frequency translating circuit according to claim 6 in. which the impedance element comprises a resistance and a condenser in parallel.

8. A frequency translating circuit according to claim 6 in which the means for effec' tively preventing grid current modulation comprises means for giving to the input circuit an impedance which is substantially zero for the products of grid current modulation.

9. A frequency translating circuit according to claim 6 having an output circuit the impedance of which is substantially equal to the output impedance of the discharge devlces for the side band or signaling currents to be transmitted and substantially zero for the "frequency of the incoming waves, carrier wave, the side band to be suppressed, and the important harmonics of those frequencies including the higher order modulation products.

10. A frequency translating circuit according to claim 6 in Which the anodes of said devices are directly connected together and having an output circuit, one terminal of Which is connected to the anodes and another terminal of which is connected to the cathodes.

11. A frequency translating device comprising a pair of electric discharge devices each having an anode, a cathode and a control electrode, the anode and cathode of one of said devices being directly connected to the corresponding electrode of the other device, an output circuit connected between the anodes and cathodes of said device's, an input circuit for said devices having a common portion connected to the cathodes and a portion individual to each of the control electrodes, a separate source of carrier Waves and a source of signaling waves connected to said input circuit, and an impedance element for applying to the control electrodes an effective negative grid biasing potential proportional to the potent-ial drop across said element produced by the carrier current. I

12. A frequency translating circuit comprising a pair of electric discharge devices each having an anode, a cathode and a control electrode, the anode and cathode of one device being directly connected to the corresponding element of the other device, an output circuit connected between the anodes and cathodes, an input circuit having a com- Inon portion connected to the cathodes of said devices and a portion individual to the control electrode of each of said devices, a separate source of carrier waves, a source of signaling Waves, means for connecting said sources of carrier and signaling Waves in conjugate relation to impress the Waves upon the control electrodes Without the interaction ot one source upon the other, and an impedance element in said input circuit for impressing upon the control electrodes an effective negative biasing potential proportional to the voltage of the carrier wave.

13. In combination, a source of carrier current, a source of signaling current, a fre quency translating circuit comprising an electric discharge device having an anode, a cathode and a control electrode and input and output circuits for said device, connections from said sources to said input circuit, an impedance in said input circuit for maintaining a biasing potential on the input electrode proportional to the potential drop across said impedance produce-d by current from said carrier source whereby the space current ofsaid device is interrupted.

In Witness whereof, I hereunto subscribe my name this 29th day of November, A. D. 1926.

HAROLD s. BLACK. 

